The use of carbon fiber reinforced plastic (“CFRP”) materials, otherwise known as carbon fiber composites, for structural members is increasing in commercial airplanes because of the higher strength-to-weight and stiffness-to-weight ratios afforded by carbon fiber composites as compared to traditional aluminum structures.
A lightning strike to an aircraft causes a high electric current, which can typically be of the order of 100,000 amps, to flow through the aircraft frame. In a carbon fiber composite structure, which is approximately 2000 times more resistive than aluminum, the carbon fiber plies act as very high resistance conductors and the resin between the plies acts as highly capacitive dielectric layers so that lightning striking the carbon fiber composite results in an increasing potential difference produced across the ply structure but no readily available electrically conductive path for discharging the current. The current therefore tends to concentrate at the fasteners between the skin panels and the aircraft substructure, since the fasteners are generally made of highly conductive alloys for strength. When the lightning energy is unable to dissipate at a fast enough rate, arcing, sparks and other unwanted effects can occur.
One current method for dissipating electric current that flows through a structure comprising a composite skin panel which is secured to a composite inner substructure with fasteners includes the use of conductive foil (e.g. copper foil). Specifically, the fasteners also secure an electrically conductive layer, such as a copper foil grid in the form of a plurality of strips, and optionally a fiberglass ply layer to the skin panel, such that the electrically conductive layer is considered an integral part of the skin. This design diverts an electrical current such as from a lightning strike away from the fasteners and along the surface of the wing structure away from underlying substructures.
The current solution to repairing the altered structure includes a hot bond repair that involves patching the area with a new electrically conductive layer using a film adhesive and solid conductive (e.g. copper) foil, which is installed either with a heat blanket or autoclave cure process. Although this solution is effective, it has some drawbacks. For instance, the hot bond repair requires taking the aircraft out of service, bringing it to a service center, and draining all fuel out of the fuel tanks, in order to perform the hot-bond repair. Only after fuel drainage can technicians apply the patch, and then must wait for the patch to cure, which can take an additional 4-8 hours, depending on the material. This approach is appropriate for in-factory repairs or repairs during regularly scheduled aircraft maintenance, when the aircraft is intended to be out of service, but a quicker solution is more desirable for in-service repairs in order to allow the airplane to get quickly back into service until a permanent repair can be scheduled.
There thus exists a need for an inexpensive and robust technique for temporarily repairing outer composite aircraft skins that utilize copper foil along fastener rows for lightning strike protection that have been altered by lightning strikes or in some other way, in order to maintain the desired lightning prevention system to help divert lightning currents away from substructures.